Image forming apparatus and image forming method in controlling electronic resistance values for image transfer

ABSTRACT

An image forming apparatus includes an image forming part having a transfer part including a transfer member and a rotation member to face the transfer member, and the transfer part to transfer a developer with a transfer voltage to a recording medium of which a front surface is with a transfer region and a non-transfer region, in the transfer target region where the developer is to be disposed and in the non-transfer region where the developer is not to be disposed; a measuring part that measures a first electric resistance value and a second electric resistance value, the first electric resistance value being defined as in a state in which the transfer region is not present between the transfer member and the rotation member, and the second electric resistance value being defined as in another state in which the transfer region is present therebetween; and a control part that determines the transfer voltage value in the transfer part based on the first electric resistance value and the second electric resistance value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC 119 to Japanese PatentApplication No. 2015-144044 filed on Jul. 21, 2015, the entire contentswhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus and an imageforming method for forming images.

BACKGROUND

In some image forming apparatuses, a transfer part transfers a tonerimage to a recording medium. For example, Patent Document 1 discloses animage forming apparatus configured to determine a transfer voltage valuebased on a transfer voltage value in a state in which no recordingmedium is present at a transfer part.

RELATED ART

[Patent Document 1] Japanese Unexamined Patent Application PublicationNo. 2014-066919

In the meantime, in an image forming apparatus, it is desired that theimage quality be high, and further improvements in the image quality areexpected.

The present invention was made in view of the aforementioned problems,and aims to provide an image forming apparatus and an image formingmethod capable of improving the image quality.

SUMMARY

An image forming apparatus includes: an image forming part having atransfer part including a transfer member, and a rotation memberarranged so as to face the transfer member, and the transfer part beingconfigured to transfer a developer with a transfer voltage to arecording medium of which a front surface is composed with a transferregion and a non-transfer region arranged between the transfer memberand the rotation member, in the transfer target region where thedeveloper is to be disposed and in the non-transfer region where thedeveloper is not to be disposed; a measuring part that measures a firstelectric resistance value and a second electric resistance value, thefirst electric resistance value being defined as a resistance valuebetween the transfer member and the rotation member in a state in whichthe transfer region of the recording medium is not present between thetransfer member and the rotation member, and the second electricresistance value being defined as another resistance value between thetransfer member and the rotation member in another state in which thetransfer region of the recording medium is present between the transfermember and the rotation member; and a control part that determines thetransfer voltage value in the transfer part based on the first electricresistance value and the second electric resistance value.

An image forming method performed with a transfer part includesmeasuring, in the transfer part having a transfer member and a rotationmember arranged so as to face the transfer member and configured totransfer a developer with a transfer voltage to a recording medium ofwhich a surface is composed with a transfer region and a non-transferregion arranged between the transfer member and the rotation member, afirst electric resistance value between the transfer member and therotation member in a state in which the transfer region of the recordingmedium is not present between the transfer member and the rotationmember; measuring a second electric resistance value between thetransfer member and the rotation member in a state in which the transferregion of the recording medium is present between the transfer memberand the rotation member; and determining the transfer voltage value inthe transfer part based on the first electric resistance value and thesecond electric resistance value.

According to the image forming apparatus and the image forming method ofthe present invention, since the transfer voltage is determined based onthe first electric resistance value in a state in which a transferregion of the recording medium is not present between the transfermember and the rotation member and the second electric resistance valuein a state in which a transfer region of the recording medium is presentbetween the transfer member and the rotation member, the image qualitycan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing one configuration example of animage forming apparatus according to a first embodiment of the presentinvention.

FIG. 2 is an explanatory view showing one configuration example of an IDunit shown in FIG. 1.

FIG. 3 is an explanatory view showing one configuration example of arecording medium shown in FIG. 1.

FIG. 4 is a table showing one characteristic example of a labeldetection sensor shown in FIG. 1.

FIG. 5 is a block diagram showing one configuration example of a controlmechanism of the image forming apparatus shown in FIG. 1.

FIG. 6 is an explanatory view showing a supply of transfer voltage to atransfer part shown in FIG. 1.

FIG. 7 is a table showing one example of a current table shown in FIG.5.

FIG. 8 is a table showing one example of a voltage table shown in FIG.5.

FIG. 9 is a flowchart showing one operational example of the imageforming apparatus shown in FIG. 1.

FIG. 10 is a flowchart showing one example of an operation to obtainelectrical characteristics of the transfer part shown in FIG. 9.

FIG. 11 is a flowchart showing one example of an operation to calculatea first transfer voltage shown in FIG. 9.

FIG. 12 is an explanatory view showing one operational example of thetransfer part shown in FIG. 1.

FIG. 13 is a flowchart showing one example of an operation to calculatea medium resistance value shown in FIG. 9.

FIG. 14 is an explanatory view showing an example an image formingresult of the image forming apparatus shown in FIG. 1.

FIG. 15 is a flowchart showing one example of an operation to calculatea second transfer voltage shown in FIG. 1.

FIG. 16 is a block diagram showing one configuration example of acontrol mechanism of an image forming apparatus according to a modifiedexample.

FIG. 17A is an explanatory view showing one operational example of anangle management part shown in FIG. 16.

FIG. 17B is another explanatory view showing one operational example ofan angle management part shown in FIG. 16.

FIG. 17C is another explanatory view showing one operational example ofan angle management part shown in FIG. 16.

FIG. 17D is another explanatory view showing one operational example ofan angle management part shown in FIG. 16.

FIG. 18 is a flowchart showing one example of an operation to calculatea medium resistance value in an image forming apparatus according to amodified example.

FIG. 19 is an explanatory view showing one configuration example of arecording medium according to a second embodiment.

FIG. 20 is a block diagram showing one configuration example of acontrol mechanism of an image forming apparatus according to a secondembodiment.

FIG. 21 is a flowchart showing one operational example of the imageforming apparatus shown in FIG. 20.

FIG. 22 is a flowchart showing one example of an operation to calculatea medium resistance value and a label width shown in FIG. 21.

FIG. 23 is a flowchart showing one example of an operation to calculatea second transfer voltage shown in FIG. 21.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, some embodiments of the present invention will be describedin detail with reference to the drawings. The description will be donein the following order.

1. Embodiment 1 (an example in which a label width W is constant)

2. Embodiment 2 (an example in which a label width W is not constant)

1. Embodiment 1 [Configuration Example]

FIG. 1 shows one configuration example of an image forming apparatus(image forming apparatus 1) according to a first embodiment of thepresent invention. The image forming apparatus 1, for example, functionsas a printer for forming an image using an electrographic method on arecording medium made of a rolled sheet.

The image forming apparatus 1 includes five ID (Image Drum) units 4 (4Y,4M, 4C, 4K, 4W), five exposure devices 6 (6Y, 6M, 6C, 6K, 6W), fiveprimary transfer rollers 7 (7Y, 7M, 7C, 7K, 7W), a transfer belt 11, adrive roller 12, an idler roller 13, a secondary transfer backup roller31, and a reverse bending roller 15.

The five ID units 4 are each configured to form a toner image.Specifically, the ID unit 4Y is configured to form a yellow (Y) tonerimage, the ID unit 4M is configured to form a magenta (M) toner image,the ID unit 4C is configured to form a cyan (C) toner image, the ID unit4K is configured to form a black (K) toner image, and the ID unit 4W isconfigured to form a white (W) toner image. The ID units 4Y, 4M, 4C, 4K,and 4W are arranged in the carrying direction F in this order.

FIG. 2 shows one configuration example of the ID unit 4. The ID unit 4is equipped with a photosensitive body 41, a charge roller 42, adevelopment roller 43, a supply roller 44, a toner accommodation section45, and a toner blade 46.

The photosensitive body 41 is a member for carrying an electrostaticlatent image on the surface (surface layer portion). The photosensitivebody 41 rotates counterclockwise in this example by a power transmittedfrom an unillustrated photosensitive body motor. The photosensitive body41 is charged by the charge roller 42. Further, the photosensitive body41 of the ID unit 4Y is exposed by the exposure device 6Y, thephotosensitive body 41 of the ID unit 4M is exposed by the exposuredevice 6M, the photosensitive body 41 of the ID unit 4C is exposed bythe exposure device 6C, the photosensitive body 41 of the ID unit 4K isexposed by the exposure device 6K, and the photosensitive body 41 of theID unit 4W is exposed by the exposure device 6W. In this way, anelectrostatic latent image is formed on the surface of each of thephotosensitive bodies 41.

The charge roller 42 is a member for charging the surface (surface layerportion) of the photosensitive body 41 to a negative voltage, forexample.

The charge roller 42 is arranged so as to be in contact with the surface(circumferential surface) of the photosensitive body 41, and rotatesclockwise in this example according to the rotation of thephotosensitive body 41. As will be explained later, a predeterminedvoltage is applied to the charge roller 42 by a high voltage powersupply part 56.

The development roller 43 is a member for carrying a toner charged to anegative voltage to the surface. The development roller 43 is arrangedso as to be in contact with the surface (circumferential surface) of thephotosensitive body 41 and rotates clockwise in this example by a powertransmitted from an unillustrated photosensitive body motor. In each ofthe photosensitive bodies 41, a toner image according to anelectrostatic latent image is formed (developed) by the toner suppliedfrom the development roller 43. As will be explained later, apredetermined voltage is applied to the development roller 43 by a highvoltage power supply part 56.

The supply roller 44 is a member configured to charge the toner storedin the toner accommodation section 45 to a negative voltage and supplyit to the development roller 43. The supply roller 44 is arranged so asto be in contact with the surface (circumferential surface) of thedevelopment roller 43 and rotates clockwise in this example by a powertransmitted from an unillustrated photosensitive body motor. With this,in the ID unit 4, friction is generated between the surface of thesupply roller 44 and the surface of the development roller 43 and as aresult, the toner is charged by the so-called frictional charge. As willbe explained later, a predetermined voltage is applied to the supplyroller 44 by the high voltage power supply part 56.

The toner accommodation section 45 is configured to store a toner.Specifically, the toner accommodation section 45 in the ID unit 4Ystores a yellow (Y) toner, the toner accommodation section 45 in the IDunit 4M stores a magenta (M) toner, the toner accommodation section 45in the ID unit 4C stores a cyan (C) toner, and the toner accommodationsection 45 in the ID unit 4K stores a black (K) toner and the toneraccommodation section 45 in the ID unit 4W stores a white (W) toner.

The toner blade 46 is a member arranged so as to be in contact with thesurface of the development roller 43 to form a layer consisting of atoner (toner layer) on the surface of the development roller 43 andregulate (control, adjust) the thickness of the toner layer. The tonerblade 46 is, for example, a plate-like elastic member (flat spring) madeof stainless steel, etc., and the front edge part of the toner blade 46is arranged so as to be in contact with the surface of the developmentroller 43. As will be explained later, a predetermined voltage isapplied to the toner blade 46 by the high voltage power supply part 56.

The five exposure devices 6 x (FIG. 1) are members each configured toirradiate, e.g., 600 dpi spot light on the photosensitive body 41 ofeach of the five ID units 4. Specifically, the exposure device 6Y is amember configured to irradiate a spot light on the photosensitive body41 of the ID unit 4Y, the exposure device 6M is a member configured toirradiate a spot light on the photosensitive body 41 of the ID unit 4M,the exposure device 6C is a member configured to irradiate a spot lighton the photosensitive body 41 of the ID unit 4C, the exposure device 6Kis a member configured to irradiate a spot light on the photosensitivebody 41 of the ID unit 4K, and the exposure device 6W is a memberconfigured to irradiate a spot light on the photosensitive body 41 ofthe ID unit 4W. With this, these photosensitive bodies 41 are exposed bythe respective exposure devices 6. As a result, an electrostatic latentimage is formed on the surface of each of the photosensitive bodies 41.

The five primary transfer rollers 7 each are a member configured toelectrostatically transfer a toner image formed by the respective fiveID units 4 on the target transfer face of the transfer belt 11. Theprimary transfer roller 7Y is arranged so as to face the photosensitivebody 41 of the ID unit 4Y via the transfer belt 11, the primary transferroller 7M is arranged so as to face the photosensitive body 41 of the IDunit 4M via the transfer belt 11, the primary transfer roller 7C isarranged so as to face the photosensitive body 41 of the ID unit 4C viathe transfer belt 11, the primary transfer roller 7K is arranged so asto face the photosensitive body 41 of the ID unit 4K via the transferbelt 11, and the primary transfer roller 7W is arranged so as to facethe photosensitive body 41 of the ID unit 4W via the transfer belt 11.As will be explained later, a predetermined voltage is applied to eachof the primary transfer rollers 7 by the high voltage power supply part56. With this, in the image forming apparatus 1, the toner image formedby each of the ID units 4 is transferred (primary transferred) onto thetarget transfer face of the transfer belt 11.

The transfer belt 11 is, for example, an endless elastic beltconstituted by a high-resistance semiconductive plastic film. Thetransfer belt 11 is extended (stretched) by the drive roller 12, theidler roller 13, the secondary transfer backup roller 31, and thereverse bending roller 15. Further, the transfer belt 11 circularlyturns in the carrying direction F according to the rotation of the driveroller 12. At that time, the transfer belt 11 passes between the ID unit4Y and the primary transfer roller 7Y, between the ID unit 4M and theprimary transfer roller 7M, between the ID unit 4C and the primarytransfer roller 7C, between the ID unit 4K and the primary transferroller 7K, and between the ID unit 4W and the primary transfer roller7W, and turns circularly.

The drive roller 12 is configured to circularly rotate the transfer belt11. In this example, the drive roller 12 is arranged on the upstreamside of the five ID units 4 in the carrying direction F and rotatesclockwise in this example by a power transmitted from an unillustratedtransfer belt motor. With this, the drive roller 12 is configured tocircularly rotate the transfer belt 11 in the carrying direction F.

The idler roller 13 is configured to be driven to rotate clockwise inthis example according to the circular rotation of the transfer belt 11.In this example, the idler roller 13 is arranged on the downstream sideof the five ID units 4 in the carrying direction F.

The secondary transfer backup roller 31 is configured to be driven torotate clockwise in this example according to the circular rotation ofthe transfer belt 11. The secondary transfer backup roller 31, in thisexample, is made of metal and is electrically grounded. The secondarytransfer backup roller 31 is, as will be explained later, arranged so asto face the secondary transfer roller 32 (explained later) sandwichingthe carrying path 20 for carrying the recording medium 9 and thetransfer belt 11. The secondary transfer backup roller 31, together withthe secondary transfer roller 32, constitutes a transfer part 30.

The reverse bending roller 15 is driven to rotate counterclockwise inthis example according to the circular rotation of the transfer belt 11.The reverse bending roller 15, in this example, is arranged between thedrive roller 12 and the secondary transfer backup roller 31, on theoutside of the path circularly rotating the transfer belt 11.

Further, the image forming apparatus 1 is equipped with a rolled sheetfeeder 21, a medium detection sensor 22, carrying rollers 23, a cuttingpart 24, a label detection sensor 25, carrying rollers 26, a secondarytransfer roller 32, ejection sensors 27 and 28, a fuser 60, and ejectionrollers 29. These members are arranged along the carrying path 20 forcarrying the recording medium 9.

In the rolled sheet feeder 21, a recording medium 9, which is a rolledsheet, is set.

FIG. 3 shows one configuration example of the recording medium 9. Therecording medium 9 includes labels 9 a and a mount 9 b. The labels 9 acan be peeled off the mount 9 b and pasted on various things. The labelwidth of the label 9 a is shown by “W”. The image forming apparatus 1 isconfigured to form an image on the label 9 a. In this example, thelabels 9 a are arranged side by side in the longitudinal direction ofthe recording medium 9.

The medium detection sensor 22 is a sensor for detecting that therecording medium 9 is being supplied to the carrying path 20 from therolled sheet feeder 21. The carrying rollers 23 are constituted by apair of rollers sandwiching the carrying path 20 and configured to carrythe recording medium 9 so that the recording medium 9 supplied from therolled sheet feeder 21 reaches an appropriate position at an appropriatetiming. The cutting part 24 is configured to cut the recording medium 9which is a rolled sheet. The cutting part 24 is configured to cut therecording medium 9, for example, when the power source of the imageforming apparatus 1 is turned on or when a user performs an operation.

The label detection sensor 25 is an optical sensor for detecting that amedium has passed through. Especially, the sensor is directed to detectthe label 9 a, which is placed, The label detection sensor 25 includes alight emitting part 25 a and a light receiving part 25 b. The lightemitting part 25 a is configured to irradiate light and the lightreceiving part 25 b is configured to receive the light emitted from thelight emitting part 25 a. The light emitting part 25 a and the lightreceiving part 25 b are arranged so as to face with each other acrossthe carrying path 20. With this configuration, the label detectionsensor 25 detects that the recording medium 9 has passed through basedon the amount of light received by the light receiving part 25 b. Atthat time, the label detection sensor 25 can detect whether the label 9a has passed or whether the mount 9 b has passed.

FIG. 4 shows one example of output voltages (detected voltages Vdet) ofthe light receiving part 25 b of the label detection sensor 25. Whenthere is no recording medium 9 between the light emitting part 25 a andthe light receiving part 25 b, the amount of light received by the lightreceiving part 25 b is large and as a result, the detected voltage Vdetbecomes high. Further, when there is the mount 9 b between the lightemitting part 25 a and the light receiving part 25 b, since a part ofthe light irradiated from the light emitting part 25 a is blocked by themount 9 b, the amount of light received by the light receiving part 25 bdecreases and as a result, the detected voltage Vdet becomes low.Further, when there is the label 9 a and the mount 9 b between the lightemitting part 25 a and the light receiving part 25 b, the amount oflight received by the light receiving part 25 b further decreases and asa result, the detected voltage Vdet becomes even lower. In this way, inthe image forming apparatus 1, the passing of the recording medium 9 canbe detected based on the detected voltage Vdet.

The carrying rollers 26 are constituted by a pair of rollers sandwichingthe carrying path 20 and configured to carry the recording medium 9along the carrying path 20.

The secondary transfer roller 32 is a member for transferring the tonerimage on the target transfer face of the transfer belt 11 to the targettransfer face of the recording medium 9. The secondary transfer roller32 includes, for example, a shaft 32 a made of metal and asemiconductive urethane rubber layer 32 b covering the outercircumference (surface) of the shaft. The secondary transfer roller 32is arranged so as to face the secondary transfer backup roller 31sandwiching the transfer belt 11 and the carrying path 20. The secondarytransfer roller 32, together with the secondary transfer backup roller31, constitutes a transfer part 30. To the shaft 32 a of the secondarytransfer roller 32, as will be explained later, a positive transfervoltage Vtr is supplied via a resistance element 39 by a voltagegeneration part 56 a. With this, in the image forming apparatus 1, thetoner image on the target transfer face of the transfer belt 11 istransferred (secondary transferred) onto the target transfer face of therecording medium 9.

The ejection sensor 27 is a sensor for detecting that the recordingmedium 9 has passed the transfer part 30.

The fuser 60 is a member configured to fuse the toner image transferredonto the recording medium 9 to the recording medium 9 by applying heatand pressure to the recording medium 9. The fuser 60 includes a heatroller 61, a pressure application roller 62, and a temperature sensor63. The heat roller 61 is a member, for example, including a heater suchas a halogen lamp inside to apply heat to the toner on the recordingmedium 9. The pressure application roller 62 is a member arranged so asto form a press-contact part between it and the heat roller 61, andconfigured to apply pressure to the toner on the recording medium 9. Thetemperature sensor 63 is configured to detect the surface temperature ofthe heat roller 61 and the pressure application roller 62. With this, inthe fuser 60, the toner on the recording medium 9 is heated, melted, andpressed. As a result, the toner image is fused on the recording medium9.

The ejection sensor 28 is a sensor for detecting that the recordingmedium 9 has passed the fuser 60. The ejection roller 29 is a memberconstituted by a pair of rollers sandwiching the carrying path 20 andconfigured to eject the recording medium 9 to the outside of the imageforming apparatus 1.

FIG. 5 shows one example of a control mechanism of the image formingapparatus 1. The image forming apparatus 1 includes an interface part51, a temperature sensor 52, a humidity sensor 53, a motor drive part54, an exposure control part 55, a high voltage power supply part 56, alabel detection part 50, a computation part 57, a memory part 58, and acontrol part 59.

The interface part 51 is configured to, for example, receive print datafrom an unillustrated host computer and perform exchanges of variouscontrol signals with the host computer. The temperature sensor 52 isconfigured to detect the environmental temperature Ta of the imageforming apparatus 1. The humidity sensor 53 is configured to detect theenvironmental humidity Ha of the image forming apparatus 1. The motordrive part 54 is configured to control the operation of each motor inthe image forming apparatus 1. With this, the motor drive part 54 isconfigured to rotate each of the photosensitive bodies 41, the driveroller 12, the carrying rollers 23 and 26, the heat roller 61, and theejection rollers 29. The exposure control part 55 is configured tocontrol the exposure operation of each exposure device 6.

The high voltage power supply part 56 supplies voltage to the chargeroller 42, the development roller 43, the supply roller 44, the tonerblade 46 of each of the ID units 4, each of the primary transfer rollers7, and the secondary transfer roller 32 of the transfer part 30. Thehigh voltage power supply part 56 includes a voltage generation part 56a and a current measuring part 56 b. The voltage generation part 56 agenerates a transfer voltage Vtr and supplies the transfer voltage Vtrto the shaft 32 a of the secondary transfer roller 32 via a resistanceelement 39 (which will be explained later). The current measuring part56 b is configured to measure the transfer current Itr in the transferpart 30.

FIG. 6 shows an operation of supplying the transfer voltage Vtr to thetransfer part 30. The output terminal of the voltage generation part 56a is connected to the shaft 32 a of the secondary transfer roller 32 viathe resistance element 39. The resistance element 39 has, for example, aresistance value R of a few MΩ and is provided to control the currentflowing in the transfer part 30. The ground terminal of the voltagegeneration part 56 a is grounded via the current measuring part 56 b.

When the transfer part 30 transfers the toner image on the transfer belt11 to the recording medium 9, the voltage generation part 56 a generatesa transfer voltage Vtr and supplies it to the secondary transfer roller32 via the resistance element 39. With this, the transfer current Itrflows through the resistance element 39, the shaft 32 a, the urethanerubber layer 32 b, the recording medium 9, the transfer belt 11, and thesecondary transfer backup roller 31 in this order. At that time, sincethe resistance value of each of these elements change due to, forexample, the temperature and the humidity, the current value of thetransfer current Itr changes, and as a result, the transfercharacteristics of the toner image of the transfer part 30 may change.In the image forming apparatus 1, as will be explained later, thetransfer voltage Vtr is determined so that the current density of thecurrent flowing through the recording medium 9 and the potentialdifference between the voltage of the surface of the recording medium 9and the voltage of the back surface of the recording medium 9 becomeapproximately constant regardless of the temperature and the humidity.With this, in the image forming apparatus 1, excellent transfercharacteristics can be obtained regardless of, for example, thetemperature and the humidity.

The label detecting part 50 is configured to detect whether or not thelabel 9 a has passed the label detection sensor 25 based on the detectedvoltage Vdet of the label detection sensor 25. Further, the labeldetection part 50 also has a function of detecting the position of thelabel 9 a on the carrying path 20.

As will be explained later, the computation part 57 is configured toobtain the transfer voltage Vtr based on the environmental temperatureTa, the environmental humidity Ha, and the value of current flowingthrough the transfer part 30.

The memory part 58 is a nonvolatile memory and is configured to store acurrent table 58 a and the voltage table 58 b.

FIG. 7 shows one example of the current table 58 a. The current table 58a shows the current density (medium current density Jp) of the currentflowing through the recording medium 9 at which the transfer part 30 iscapable of transferring the toner image to the label 9 a of therecording medium 9 in an excellent manner. The medium current density Jpis a current value per unit length in the widthwise direction of therecording medium 9 (depth direction in FIG. 1) and the unit of themedium current density Jp is μA/mm in this example. The current table 58a includes medium current densities Jp at various temperatures andhumidity.

FIG. 8 shows one example of the voltage table 58 b. The voltage table 58b shows a potential difference (medium voltage density Vp) between thevoltage of the surface of the recording medium 9 and the voltage of theback surface of the recording medium 9 at which the transfer part 30 iscapable of transferring the toner image to the label 9 a of therecording medium 9 in an excellent manner. The unit of the mediumvoltage Vp is kV in this example. The voltage table 58 b includes mediumvoltages Vp at various temperatures and humidity.

FIGS. 7 and 8 are examples and are not limited to the values. That is,for example, the value of the medium current density Jp and the value ofthe medium voltage Vp change according to the print speed, for example.Further, for example, the medium current density Jp and the mediumvoltage Vp can be set by further dividing all of the temperature rangeand all of the humidity range, or the medium current density Jp and themedium voltage Vp can be set by dividing them more roughly. Further, aplurality of current tables 58 a and voltage tables 58 b may be providedand for example, one among the plurality of current tables 58 a and oneamong the plurality of voltage tables 58 b may be selected according tothe type of recording medium 9 to be used.

The control part 59 is configured to control the overall operation ofthe image forming apparatus 1 by controlling the operations of varioussensors shown in each of these blocks and in FIG. 1.

Further, the computation part 57 and the control part 59 may beconfigured to include, for example, a microprocessor, a ROM (Read OnlyMemory), a RAM (Random Access Memory), input/output ports, a timer, etc.

Here, the secondary transfer roller 32 corresponds to one specificexample of a “transfer member” of the present invention. The secondarytransfer backup roller 31 corresponds to one specific example of a“rotation member” of the present invention. The toner corresponds to onespecific example of a “developer” of the present invention. The five IDunits 4, the five exposure devices 6, the five primary transfer rollers7, the transfer belt 11, and the transfer part 30 correspond to specificexamples of the “image forming part” of the present invention. Thecurrent measuring part 56 b corresponds to one specific example of the“measuring part” of the present invention. The computation part 57 andthe control part 59 correspond to one specific example of the “controlpart” of the present invention. The temperature sensor 52 and thehumidity sensor 53 correspond to one specific example of the“environment detection part” of the present invention. The region of thelabel 9 a of the recording medium 9 corresponds to one specific exampleof the “transfer region” of the present invention, and the region otherthan the label 9 a of the recording medium 9 corresponds to one specificexample of the “non-transfer region” of the present invention.

The temperature sensor 52 detects an environmental temperature thatincludes various types of temperature indicating a working conditionaround the apparatus. Not only the apparatus's temperature but atemperature of a room in which the apparatus is placed also areavailable. The humidity sensor 53 detects an environmental humidity thatincludes various types of humidity indicating a working condition aroundthe apparatus. For example, in addition to a humidity inside theapparatus, a room humidity as well is available. These sensors may beprovided directly with the apparatus but may be equipped at anywhereother than the apparatus. Such a sensor, which is equipped remotely fromthe apparatus, can send sensed information to the apparatus with a cableor without a cable (or wirelessly). Wireless communication techniques,such as infrared transmission, WiFi, or Bluetooth, are available toachieve these communication between them. In order to collect theseenvironmental temperature and humidity, multiple sensors can be used atmultiple locations.

[Operations and Functions]

Next, operations and functions of the image forming apparatus 1 of thisembodiment will be described.

(Summary of General Operations)

First, with reference to FIGS. 1, 2, and 5, the summary of the generaloperations of the image forming apparatus 1 will be described. In theimage forming apparatus 1, after the control part 59 receives print datafrom a host computer via the interface part 51, the control part 59first controls the fuser 60 and operates the heater of the heat roller61.

When the temperature of the fuser 60 detected by the temperature sensor63 reaches a temperature suitable for the fusing operation, the controlpart 59 controls the motor drive part 54 to rotate the photosensitivebody 41 of each of the ID units 4. Then, the control part 59 controlsthe linear velocity of the photosensitive body 41 so that it becomesapproximately the same as the carrying speed of the recording medium 9at the time of printing. At the same time, the control part 59 controlsthe motor drive part 54 to rotate the drive roller 12, the carryingrollers 23 and 26, the heat roller 61, and the ejection rollers 29.Then, the control part 59 controls the carrying speed so that it becomesapproximately the same as the carrying speed of the recording medium 9at the time of printing.

Further, the control part 59 starts to rotate the photosensitive body 41in this way, and also controls the high voltage power supply part 56 toapply a negative voltage (for example, −1,150V) to the charge roller 42.As a result, the photosensitive body 41 is charged to a negative voltage(for example, −700V). Further, the control part 59 controls the highvoltage power supply part 56 to apply a negative voltage (for example,−300V) to the development roller 43. Then, in the ID unit 4, when thephotosensitive body 41 rotates and the negatively charged portion of thephotosensitive body 41 reaches the nip part between the photosensitivebody 41 and the primary transfer roller 7, the ID unit 4 becomes in aprintable state.

Next, the control part 59 controls the motor drive part 54 to carry therecording medium 9 to a predetermined position from the rolled sheetfeeder 21 along the carrying path 20 based on the detection result ofthe medium detection sensor 22. Then, the control part 59, based on thedetection result of the label detection sensor 25, obtains the timing inwhich the front end of the recording medium 9 reaches the nip partbetween the secondary transfer backup roller 31 and the secondarytransfer roller 32 in the transfer part 30.

Next, the control part 59, based on the print data, generates image datato be formed by each of the ID units 4. Then, the control part 59,considering the timing in which the front end of the recording medium 9reaches the nip part, controls the exposure control part 55 at apredetermined timing to expose the photosensitive body 41 of each of theID units 4 with each of the exposure devices 6. With this, in each ofthe ID units 4, the voltage of the exposed portion among the surface ofthe photosensitive body 41 becomes about 0 V and an electrostatic latentimage is formed.

The control part 59 controls the high voltage power supply part 56 toapply a negative voltage (for example, −400V) to the supply roller 44and apply a negative voltage to the toner blade 46 (for example, −400V).With this, the supply roller 44 charges the toner to a negative voltageand supplies the toner to the development roller 43. The toner suppliedto the development roller 43 is regulated by the toner blade 46 and ischarged to a negative voltage. Since the electrical potential of theexposed portion of the surface of the photosensitive body 41 is about 0V, the toner charged to a negative voltage on the development roller 43moves to the exposed portion on the surface of the photosensitive body41 from the development roller 43 due to Coulomb force. With this, onthe photosensitive body 41, the toner image is developed as a visibleimage.

The control part 59 controls the high voltage power supply part 56 toapply a positive voltage (for example, +1,500V) to each of the primarytransfer rollers 7. With this, the toner charged to a negative voltageon the photosensitive body 41 moves to the transfer belt 11 from thephotosensitive body 41 due to Coulomb force.

The control part 59 controls the high voltage power supply part 56 tosupply the positive transfer voltage Vtr determined by the computationpart 57 to the secondary transfer roller 32 via the resistance element39. With this, the toner charged to a negative voltage on the transferbelt 11 moves to the recording medium 9 from the transfer belt 11 due toCoulomb force.

The toner on the recording medium 9 is heated, melted, and pressed bythe fuser 60. As a result, the toner image is fused on the recordingmedium 9.

(Detail Operations)

Next, operations of determining the transfer voltage Vtr applied to thesecondary transfer roller 32 will be described in detail.

FIG. 9 shows a flowchart of the operations to determine the transfervoltage Vtr. First, after turning on the power, the image formingapparatus 1 obtains the electrical characteristics of the transfer part30 in a state in which there is no recording medium 9 in the transferpart 30. Then, the image forming apparatus 1 determines the transfervoltage Vtr after receiving the print data, and starts printing. Afterthat, the image forming apparatus 1 determines a transfer voltage Vtragain when the print distance M becomes a predetermined distance Mth ormore. Hereinafter, this operation will be described in detail. The printdistance M is a distance along the carrying path 20, is defined as alength in which the sheet is carried for printing.

First, after the power of the image forming apparatus 1 is turned on,the image forming apparatus 1 obtains the electrical characteristics ofthe transfer part 30 (S1).

(Obtaining Electrical Characteristics of Transfer Part 30)

FIG. 10 shows a flowchart of the operations to obtain electricalcharacteristics of the transfer part 30.

First, the control part 59 of the image forming apparatus 1 controls thecutting part 24 to cut the recording medium 9 (S21). Then, the imageforming apparatus 1 starts the carrying operation (Step S22).Specifically, the control part 59 controls the motor drive part 54 torotate the drive roller 12, the carrying rollers 26, the heat roller 61,and the ejection rollers 29. With this, the transfer part 30 becomes ina state in which there is no recording medium 9.

Next, the image forming apparatus 1 supplies a voltage V1 to thesecondary transfer roller 32 via the resistance element 39 and detectsthe current I1 (S23). Specifically, the voltage generation part 56 a ofthe high voltage power supply part 56 generates a voltage V1 based oninstructions from the control part 59. Then, the current measuring part56 b detects the current I1 and supplies the detected result to thecontrol part 59.

Next, the image forming apparatus 1 supplies a voltage V2 that isdifferent from the voltage V1 to the secondary transfer roller 32 viathe resistance element 39, and detects the current I2 (S24).Specifically, the voltage generation part 56 a generates the voltage V2based on the instructions from the control part 59. Then, the currentmeasuring part 56 b detects the current I2 and supplies the detectedresult to the control part 59.

Further, in this example, currents I1 and I2 are each detected once, butit is not limited to that, and for example, the current I1 can bedetected a plurality of times and the average value can be obtained andthe current I2 can be detected a plurality of times and the averagevalue can be obtained.

Next, the computation part 57 of the image forming apparatus 1calculates shaft voltages Vs (Vs1, Vs2) of the shaft 32 a at the time ofsupplying a voltage in Steps S23 and S24 (S25). That is, since thevoltage generation part 56 a supplies a voltage to the secondarytransfer roller 32 via the resistance element 39, the shaft voltage ofthe shaft 32 a is different from the voltage generated by the voltagegeneration part 56 a. The shaft voltages Vs1 and Vs2 can be shown asfollows using the resistance value R of the resistance element 39.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\\left. \begin{matrix}{{{Vs}\; 1} = {{V\; 1} - {R \times I\; 1}}} \\{{{Vs}\; 2} = {{V\; 2} - {R \times I\; 2}}}\end{matrix} \right\} & (1)\end{matrix}$

The computation part 57 calculates the shaft voltages Vs1 and Vs2 usingthe aforementioned formula (1).

Next, the computation part 57 calculates a current density J (J1, J2) ofthe transfer part 30 at the time of supplying the voltage in Steps S23and S24 (S26). Here, the current densities J1 and J2 are current valuesper unit of length in the lengthwise direction (depth direction inFIG. 1) of the secondary transfer roller 32, and the unit of the currentdensities J1 and J2 are, for example, μA/mm. When the length of thesecondary transfer roller 32 is L, the current densities J1 and J2 canbe shown as follows:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\\left. \begin{matrix}{{J\; 1} = \frac{I\; 1}{L}} \\{{J\; 2} = \frac{I\; 2}{L}}\end{matrix} \right\} & (2)\end{matrix}$

The computation part 57 calculates the current densities J1 and J2 usingthe aforementioned formula (2).

Next, the computation part 57 obtains the relational expression of thecurrent density J and the shaft voltage Vs by linear approximation(S27).

The current density J can be shown as follows using the shaft voltage Vsand coefficients a and b.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\\left. \begin{matrix}{J = {{a \times {Vs}} + b}} \\{a = \frac{{J\; 2} - {J\; 1}}{{{Vs}\; 2} - {{Vs}\; 1}}} \\{b = \frac{{J\; 1 \times {Vs}\; 2} - {J\; 2 \times {Vs}\; 1}}{{{Vs}\; 2} - {{Vs}\; 1}}}\end{matrix} \right\} & (3)\end{matrix}$

The computation part 57 calculates the coefficients a and b using theshaft voltages Vs1 and V2 calculated in Step S25 (formula (1)), thecurrent densities J1 and J2 calculated in Step S26 (formula (2)) and theaforementioned formula (3).

In addition, the operation to obtain the electrical characteristics ofthe transfer part 30 (S21 to S27) may be performed at least once afterturning on the power and before the start of printing.

With the above, the flow of obtaining the electrical characteristics ofthe transfer part 30 is completed.

Next, as shown in FIG. 9, the control part 59 of the image formingapparatus 1 confirms whether or not the print data has been received(S2). If the print data has not been received (“N” in S2), the flowreturns to Step S2. Then, Step S2 is repeated until print data isreceived.

Then, when the print data is received (“Y” in S2), the image formingapparatus 1 calculates the transfer voltage Vtr (S3).

(Calculation 1 of Transfer Voltage Vtr)

FIG. 11 shows a flowchart of the operations to calculate the transfervoltage Vtr.

First, the control part 59 of the image forming apparatus 1 obtainsinformation for the label width W of the label 9 a included in the printdata and also obtains the environmental temperature Ta detected by thetemperature sensor 52 and the environmental humidity Ha detected by thehumidity sensor 53 (S31). Further, in this example, the label width W ofthe label 9 a is obtained based on the print data, but it is not limitedto that. Specifically, for example, when the image forming apparatus 1is equipped with a detector for sensing the label width W of the label 9a, the control part 59 can obtain the label width W from the detector.

Next, the computation part 57 of the image forming apparatus 1calculates the medium current density Jp and the medium voltage Vp(S32). Specifically, the computation part 57 calculates the mediumcurrent density Jp and the medium voltage Vp using the environmentaltemperature Ta and the environmental humidity Ha obtained in Step S31and utilizing the current table 58 a and the voltage table 58 b.

Next, the computation part 57 obtains a shaft voltage Vs0 capable ofrealizing the medium current density Jp and the medium voltage Vpobtained in Step S32 (S33).

FIG. 12 schematically depicts the transfer part 30 viewed from thedirection of the carrying path 20. FIG. 12 shows a case in which arecording medium 9 is present in the transfer part 30. In this drawing,for the convenience of description, the mount 9 b is not depicted. Thatis, in the following description, only the label 9 a of the recordingmedium 9 is considered with the assumption that effects of the mount 9 bis small. The recording medium 9 (label 9 a) is sandwiched between thetransfer belt 11 and the urethane rubber layer 32 b of the secondarytransfer roller 32. In the drawing, in the lengthwise direction of thesecondary transfer roller 32 (lateral direction in FIG. 12), the regionin which the label 9 a is present is shown as a region R1 and the regionin which the label 9 a is not present is shown as a region R2. In thisexample, since the secondary transfer backup roller 31 is grounded, thevoltage between the secondary transfer backup roller 31 and the shaft 32a is the shaft voltage Vs0.

Attention will be focused on the region R1. The shaft voltage Vs0 can beshown as follows.[Formula 4]Vs0=Vin+Vp  (4)

Here, the voltage Vin is a voltage occurring in the transfer belt 11 andthe urethane rubber layer 32 b. That is, the first item on the rightside shows the contribution by the transfer belt 11 and the urethanerubber layer 132 b and the second item on the right side shows thecontribution by the label 9 a of the recording medium 9. In the regionR1, the current density in the transfer belt 11 and the urethane rubberlayer 32 b is approximately the same as the current density of thecurrent flowing through the recording medium 9 (medium current densityJp). Therefore, the voltage Vin can be shown as follows using theformula (3).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack & \; \\{{Vin} = \frac{{Jp} - b}{a}} & (5)\end{matrix}$

Therefore, the shaft voltage Vs0 can be shown as follows using theformulas (4) and (5).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack & \; \\{{{Vs}\; 0} = {\frac{{Jp} - b}{a} + {Vp}}} & (6)\end{matrix}$

The computation part 57 calculates the shaft voltage Vs0 using theaforementioned formula (6).

Next, the computation part 57 calculates the transfer current Itr (S34).First, attention will be focused on the region R2. Since both thesecondary transfer backup roller 31 and the shaft 32 a are made ofmetallic, the shaft voltage Vs0 obtained while focusing on the region R1in Step S33 can also be used in the region R2.

Since no label 9 a is present in the region R2, the relationalexpression (formula (3)) for the current density J and the shaft voltageVs in a case in which there are no recording medium 9 in the transferpart 30 as obtained in Step S27 can be used. The current density Jout ofthe current flowing through the region R2 can be shown as follows usingthe formula (3).[Formula 7]Jout=a×Vs0+b  (7)

The transfer current Itr can be shown as follows using the formula (7)

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack & \; \\\begin{matrix}{{Itr} = {{{Jp} \times W} + {{Jout} \times \left( {L - W} \right)}}} \\{= {{{Jp} \times W} + {\left( {{a \times {Vs}\; 0} + b} \right) \times \left( {L - W} \right)}}}\end{matrix} & (8)\end{matrix}$

Here, the first item on the right side shows the contribution by theregion R1 and the second item on the right side shows the contributionby the region R2. The computation part 57 calculates the transfercurrent Itr using the formula (8).

Next, the computation part 57 calculates the transfer voltage Vtr to begenerated by the voltage generation part 56 a (S35). As shown in FIG. 4,the voltage generation part 56 a supplies a voltage to the shaft 32 a ofthe secondary transfer roller 32 via the resistance element 39.Therefore, the transfer voltage Vtr to be generated by the voltagegeneration part 56 a can be shown as follows.[Formula 9]Vtr=Vs0+R×Itr  (9)

Here, the first item on the right side shows the contribution by thetransfer part 30 and the second item on the right side shows thecontribution by the resistance element 39. The computation part 57calculates the transfer voltage Vtr using the shaft voltage Vs0calculated in Step S33 (formula (4)), the transfer current Itrcalculated in Step S34 (formula (8)) and the aforementioned formula (9).

With the above, the flow of the operation to calculate the transfervoltage Vtr is completed.

Next, as shown in FIG. 9, the image forming apparatus 1 starts the printoperation (S4). At that time, the voltage generation part 56 a generatesthe transfer voltage Vtr obtained in Step S3 and supplies the transfervoltage Vtr to the secondary transfer roller 32 via the resistanceelement 39 based on the instructions from the control part 59. Withthis, since the current density of the current flowing in the recordingmedium 9 can be set around the medium current density Jp and thepotential difference between the voltage of the surface of the recordingmedium 9 and the back surface of the recording medium 9 can be setaround the medium voltage Vp, excellent transfer characteristics can beobtained.

Next, the image forming apparatus 1 calculates the medium resistancevalue Rb (S5).

(Calculation of Medium Resistance Value Rb)

FIG. 13 is a flowchart of the operations to calculate the mediumresistance value Rb.

First, the medium detection sensor 22 detects the recording medium 9(S41).

Next, the label detection sensor 25 detects the label 9 a of therecording medium 9 (S42). The label detection part 50, based on thedetected result of the label detection sensor 25, hereinafter obtainsthe position of the label 9 a on the carrying path 20.

Next, the current measuring part 56 b of the image forming apparatus 1detects the current Itr1 before the label 9 a of the recording medium 9reaches the transfer part 30 (S43). That is, the image forming apparatus1 has already started the printing operation in Step S4 and the voltagegeneration part 56 a supplies the transfer voltage Vtr to the secondarytransfer roller 32 via the resistance element 39. Therefore, the currentmeasuring part 56 b detects the current Itr1 that flows according to thetransfer voltage Vtr before the label 9 a of the recording medium 9reaches the transfer part 30. Then, the current measuring part 56 bsupplies the detected result to the control part 59.

Next, the current measuring part 56 b detects the current Itr2 after thelabel 9 a of the recording medium 9 reaches the transfer part 30 (S44).Then, the current measuring part 56 b supplies the detected result tothe control part 59.

Next, the computation part 57 calculates a resistance value Rt1 of thetransfer part 30 when no label 9 a of the recording medium 9 is presentin the transfer part 30 and a resistance value Rt2 of the transfer part30 when the label 9 a of the recording medium 9 is present in thetransfer part 30 (S45). Specifically, the resistance values Rt1 and Rt2of the transfer part 30 can be shown as follows.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack & \; \\\left. \begin{matrix}{{{Rt}\; 1} = {\frac{Vtr}{{Itr}\; 1} - R}} \\{{{Rt}\; 2} = {\frac{Vtr}{{Itr}\; 2} - R}}\end{matrix} \right\} & (10)\end{matrix}$

The computation part 57 calculates the resistance values Rt1 and Rt2 ofthe transfer part 30 based on the aforementioned formulas.

Next, the computation part 57 calculates a medium resistance value Rb(S46). First, attention will be focused on the region R1. The resistancevalue Rt3 of the transfer part 30 in the region R1 can be shown asfollows using the medium resistance value Rb and the resistance valueRt1 of the transfer part 30 in a state in which no label 9 a of therecording medium 9 is present in the transfer part 30.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack & \; \\{{{Rt}\; 3} = {{Rb} + {{Rt}\; 1 \times \frac{L}{W}}}} & (11)\end{matrix}$

Here, the second item on the right side is the total resistance value ofthe resistance value of the transfer belt 11 and the resistance value ofthe urethane rubber layer 32 b in the region R1.

Next, attention will be focused on the region R2. The resistance valueRt4 of the transfer part 30 in the region R2 can be shown as followsusing the resistance value Rt3 of the transfer part 30 in the region R1and the resistance value Rt2 of the transfer part 30 in a state in whichthe label 9 a of the recording medium 9 is present in the transfer part30.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack & \; \\{{{Rt}\; 4} = \frac{{Rt}\; 2 \times {Rt}\; 3}{{{Rt}\; 2} - {{Rt}\; 3}}} & (12)\end{matrix}$

The medium resistance value Rb can be shown as follows using theformulas (11) and (12).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 13} \right\rbrack & \; \\{{Rb} = {\frac{{Rt}\; 2 \times {Rt}\; 4}{{{Rt}\; 2} - {{Rt}\; 4}} + {\frac{{Rt}\; 1 \times \left( {{{Rt}\; 4} - {{Rt}\; 2}} \right)}{{{Rt}\; 2} - {{Rt}\; 4}} \times \frac{L}{W}}}} & (13)\end{matrix}$

The computation part 57 calculates the medium resistance value Rb usingthe resistance values Rt1 and Rt2 calculated in Step S45 (formula (10)),the resistance value Rt4 calculated in Step S46 (formula (12)) and theaforementioned formula (13).

With the above, the flow of calculation of the medium resistance valueRb is completed.

Next, as shown in FIG. 9, the control part 59 of the image formingapparatus 1 confirms whether or not the print distance M in therecording medium 9 after starting printing in Step S4 is longer than thepredetermined distance Mth (for example, 1 m) (M>Mth) (S6). When theprint distance M is equal to or shorter than the predetermined distanceMth (M≦Mth) (“N” in S6), the flow returns to Step S6. Then, Step S6 isrepeated until the print distance M becomes longer than thepredetermined distance Mth.

When the print distance M is longer than the predetermined distance Mth(“Y” in S6), the control part 59 confirms whether or not the label 9 ais present in the transfer part 30 and that the image forming apparatus1 in a state in which it is not forming an image (S7).

FIG. 14 schematically shows the recording medium 9 after an image isformed. In FIG. 14, the region 91 shows a region of a label 9 a in whichan image is formed and the region 92 shows a region of the label 9 a inwhich no image is formed. The control part 59 confirms whether or notthe transfer part 30 is forming an image by transferring a toner imagelike in the region 91 or not forming an image like in the region 92. Incases where no label 9 a is present in the transfer part 30, or theimage forming apparatus 1 is not in a non-forming state (or theapparatus 1 is forming an image), the step determines No (“N” in S7),the flow returns to Step S7. Then, Step S7 is repeated until the stepdetermines YES, which is defined follow. Only where a label 9 a ispresent in the transfer part 30 and the image forming apparatus 1 is inthe non-forming state in which the apparatus 1 is not forming an image,the step determines YES.

Then, when it becomes in a state in which a label 9 a is present in thetransfer part 30 and the image forming apparatus 1 is not forming animage (“Y” in S7), the image forming apparatus 1 calculates the transfervoltage Vtr again (S8).

(Calculation 2 of Transfer Voltage Vtr)

FIG. 15 shows a flowchart of an operation to calculate the transfervoltage Vtr.

First, the current measuring part 56 b of the image forming apparatus 1detects a current Itr3 (S51). That is, at this time, the voltagegeneration part 56 a is supplying the transfer voltage Vtr to thesecondary transfer roller 32 via the resistance element 39 and a label 9a of a recording medium 9 has already reached the transfer part 30.Therefore, the current measuring part 56 b detects a current Itr3 in astate in which the label 9 a of the recording medium 9 is present in thetransfer part 30. Then, the current measuring part 56 b supplies thedetected result to the control part 59.

Next, the computation part 57 calculates a resistance value Rt5 of thetransfer part 30 in a state in which the label 9 a of the recordingmedium 9 is present in the transfer part 30 (S52). Specifically, theresistance value Rt5 of the transfer part 30 can be shown as follows.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack & \; \\{{{Rt}\; 5} = {\frac{Vtr}{{Itr}\; 3} - R}} & (14)\end{matrix}$

The computation part 57 calculates the resistance value Rt5 of thetransfer part 30 using the formula (14).

Next, the computation part 57 calculates the resistance value Rt6 of thetransfer part 30 in a state in which no label 9 a of the recordingmedium 9 is present in the transfer part 30 (S53). The resistance valueRt5 of the transfer part 30 in a state in which the label 9 a of therecording medium 9 is present in the transfer part 30 and the resistancevalue Rt6 of the transfer part 30 in a state in which no label 9 a ofthe recording medium 9 is present in the transfer part 30 have thefollowing relationship.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 15} \right\rbrack & \; \\{\frac{1}{{Rt}\; 5} = {\frac{1}{{Rb} + {{Rt}\; 6 \times \frac{L}{W}}} + \frac{1}{{Rt}\; 6 \times \frac{L}{L - W}}}} & (15)\end{matrix}$

Here, the first item on the right side shows a conductance in the regionR1 and the second item on the right side shows a conductance in theregion R2. The following formula is obtained by arranging the formula(15) about the resistance value Rt6.[Formula 16]L ² λRt6²−(L ² ×Rt5−W×L×Rb)×Rt6+W×(L−W)×Rb×Rt5=0  (16)The next formula is obtained by solving the formula (16) for theresistance value Rt6.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 17} \right\rbrack & \; \\{{{Rt}\; 6} = \frac{\begin{pmatrix}{{L^{2} \times {Rt}\; 5} - {{W \times L \times {Rb}} \pm}} \\\sqrt{\begin{matrix}{\left( {{L^{2} \times {Rt}\; 5} - {W \times L \times {Rb}}} \right)^{2} -} \\{4 \times L^{2} \times W \times \left( {L - W} \right) \times {Rb} \times {Rt}\; 5}\end{matrix}}\end{pmatrix}}{2 \times L^{2}}} & (17)\end{matrix}$

The positive value among the two values obtained using the formula (17)is the resistance value Rt6. The computation part 57 calculates theresistance value Rt6 of the transfer part 30 in a state in which nolabel 9 a of the recording medium 9 is present in the transfer part 30using the medium resistance value Rb calculated in Step S5 (formula(13)), the resistance value Rt5 calculated in Step S52 (formula (14))and the aforementioned formula (17).

Next, the computation part 57 obtains a shaft voltage Vs0 (S54).Focusing on the region R1, the shaft voltage Vs0 can be shown by theformula (4). Focusing on region R2, the voltage Vin can be shown asfollows.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 18} \right\rbrack & \; \\\begin{matrix}{{Vin} = {{Jp} \times \left( {L - W} \right) \times {Rt}\; 6 \times \frac{L}{L - W}}} \\{= {{Jp} \times {Rt}\; 6 \times L}}\end{matrix} & (18)\end{matrix}$

Therefore, the shaft voltage Vs0 can be shown as follows using theformulas (4) and (18).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 19} \right\rbrack & \; \\\begin{matrix}{{{Vs}\; 0} = {{Vin} + {Vp}}} \\{= {{{Jp} \times {Rt}\; 6 \times L} + {Vp}}}\end{matrix} & (19)\end{matrix}$

The computation part 57 calculates the shaft voltage Vs0 using theresistance value Rt6 calculated in Step S53, the medium current densityJp and the medium voltage Vp calculated in Step S32, and this formula.

Next, the computation part 57 calculates a transfer current Itr (S55).First, attention will be focused on the region R2. Since both thesecondary transfer backup roller 31 and the shaft 32 a are made ofmetallic, the shaft voltage Vs0 obtained by focusing on the region R1 inStep S54 can also be used in the region R2. The current Tout flowingthrough the region R2 can be shown as follows.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 20} \right\rbrack & \; \\{{Iout} = \frac{{Vs}\; 0}{{Rt}\; 6 \times \frac{L}{L - W}}} & (20)\end{matrix}$

Therefore, the transfer current Itr can be shown as follows using theformula (20).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 21} \right\rbrack & \; \\\begin{matrix}{{Itr} = {{{Jp} \times W} + {Iout}}} \\{= {{{Jp} \times W} + \frac{{Vs}\; 0}{{Rt}\; 6 \times \frac{L}{L - W}}}}\end{matrix} & (21)\end{matrix}$

Here, the first item on the right side shows a contribution by theregion R1 and the second item on the right side shows a contribution bythe region R2. The computation part 57 calculates the transfer currentItr using the resistance value Rt6 calculated in Step S53 (formula(17)), the shaft voltage Vs0 calculated in Step S54 (formula (19)) andthe aforementioned formula (21).

Next, the computation part 57 calculates a transfer voltage Vtr to begenerated by the voltage generation part 56 a (S56). The transfervoltage Vtr to be generated by the voltage generation part 56 a can beshown as follows.[Formula 22]Vtr=Vs0+R×Itr  (22)

The computation part 57 calculates the transfer voltage Vtr using theshaft voltage Vs0 calculated in Step S54 (formula (19)), the transfercurrent Itr calculated in Step S55 (formula (21)), and theaforementioned formula (22).

With the above, the flow of the operation to calculate the transfervoltage Vtr is completed.

The voltage generation part 56 a, during a period in which no image isbeing formed, generates a transfer voltage Vtr based on the instructionsfrom the control part 59 and supplies the transfer voltage Vtr to thesecondary transfer roller 32 via the resistance element 39. That is, thetransfer voltage Vtr is updated during a period in which no image isbeing formed. Then, after that, the image forming apparatus 1 continuesthe print operation. With this, the current density of the currentflowing through the recording medium 9 can be brought closer to themedium current density Jp, and the potential difference between thevoltage of the surface of the recording medium 9 and the voltage of theback surface of the recording medium 9 can be brought closer to themedium voltage Vp, and therefore excellent transfer characteristics canbe obtained.

In this way, when the print distance M is longer than the predetermineddistance Mth, the image forming apparatus 1 obtains the resistance valueRt5 of the transfer part 30 in a state in which the label 9 a of therecording medium 9 is present in the transfer part 30. Then, based onthe resistance value Rt5, the resistance value Rt6 of the transfer part30 in a state in which no label 9 a of the recording medium 9 is presentin the transfer part 30 is obtained, and the transfer voltage Vtr isobtained based on the resistance value Rt6. With this, the image qualitycan be improved in the image forming apparatus 1. That is, whenperforming printing continuously for a long period of time, theresistance value of the transfer part 30 may change due to thegeneration of heat, for example. In this case, for example, the currentdensity in the recording medium 9 may deviate from the desired mediumcurrent density Jp, or the potential difference between the voltage ofthe surface of the recording medium 9 and the voltage of the backsurface of the recording medium 9 may deviate from the desired mediumvoltage Vp. As a result, the transfer characteristics in the transferpart 30 deteriorate, and for example, print failure may occur, such asfading of the characters. In particular, when the recording medium 9 isa rolled sheet, since printing is performed continuously for a longperiod of time, once printing is started, printing failure occurseasily. When the print distance M is longer than the predetermineddistance Mth, the image forming apparatus 1 obtains the resistance valueRt5 of the transfer part 30 and the transfer voltage Vtr is obtainedbased on the resistance value Rt5. With this, even when printing isperformed continuously for a long period of time, the current density ofthe current flowing through the recording medium 9 can be brought closerto the medium current density Jp and the potential difference betweenthe voltage of the surface of the recording medium 9 and the voltage ofthe back surface of the recording medium 9 can be brought closer to themedium voltage Vp. As a result, excellent transfer characteristics canbe obtained and the image quality can be improved in the image formingapparatus 1.

Further, in the image forming apparatus 1, since the resistance valueRt5 of the transfer part 30 is obtained during a period in which noimage is being formed, the transfer voltage Vtr can be obtainedaccurately. That is, for example, when the resistance value Rt5 of thetransfer part 30 is obtained during a period in which the image formingapparatus 1 is forming an image, the resistance value Rt5 may beaffected by the toner since a toner exists in the transfer part 30.Therefore, when the transfer voltage Vtr is obtained based on theresistance value Rt5, for example, the current density in the recordingmedium 9 may deviate from the desired medium current density Jp, or thepotential difference between the voltage of the surface of the recordingmedium 9 and the voltage of the back surface of the recording medium 9may deviate from the desired medium voltage Vp. In the image formingapparatus 1, during the period in which no image is being formed, theresistance value Rt5 of the transfer part 30 is obtained and thetransfer voltage Vtr is obtained based on the resistance value Rt5. Withthis, the transfer voltage Vtr can be accurately obtained without beingaffected by a toner. As a result, excellent transfer characteristics canbe obtained and the image quality can be improved in the image formingapparatus 1.

Further, in the image forming apparatus 1, since the transfer voltageVtr is updated during the period in which no image is being formed, theimage quality can be improved. That is, for example, when the transfervoltage Vtr is updated when the image forming apparatus 1 is forming animage, since the transfer characteristics change largely within oneimage, the image quality may deteriorate. In the image forming apparatus1, since the transfer voltage Vtr is updated during a period in which noimage is formed, the transfer characteristics do not change largelywithin one image, so the risk of the deterioration of the image qualitycan be reduced.

[Effects]

In this embodiment, as explained above, since the resistance value in astate in which a label of the recording medium is present at thetransfer part is obtained, and the transfer voltage is obtained based onthe resistance value, the image quality can be improved even whenprinting continuously for a long period of time.

Further, in this embodiment, since the resistance value in a state inwhich a recording medium is present at the transfer part is obtainedduring a period in which no image is being formed, the transfer voltagecan be accurately obtained, and as a result, the image quality can beimproved.

Further, in this embodiment, since the transfer voltage is updatedduring a period in which no image is being formed, the image quality canbe improved.

Modified Embodiment 1-1

In the aforementioned embodiment, the toner image formed by each of theID units 4 is transferred (primary transfer) onto the target transferface of the transfer belt 11 and then the toner image on the targettransfer face of the transfer belt 11 is transferred (secondarytransfer) onto the target transfer face of the recording medium 9, butnot limited to that. Alternately, a toner image formed by each of the IDunits 4 may be directly transferred onto the target transfer face of therecording medium 9. In this case, the computation part 57 may calculateeach transfer voltage of the five transfer rollers facing each of thefive ID units 4. However, it is not limited to that, and for example,the transfer voltage may be calculated using the aforementioned methodfor only some of the five transfer rollers and the transfer voltages ofthe remaining transfer rollers may be roughly estimated using thecalculated result. Specifically, for example, the transfer voltage ofthe transfer roller arranged on the most upstream side and the transfervoltage of the transfer roller arranged on the most downstream sideamong the five transfer rollers may be calculated using theaforementioned method.

Modified Embodiment 1-2

In the aforementioned embodiment, the predetermined distance Mth is setto 1 m, for example, but it is not limited to that. That is, forexample, the value of the predetermined distance Mth may changeaccording to, for example, the print speed, the material of thesecondary transfer roller 32, etc. Therefore, for example, it ispreferably set for each type of image forming apparatus 1.

Modified Embodiment 1-3

In the aforementioned embodiment, the computation part 57 obtains thetransfer voltage Vtr when the print distance M is longer than thepredetermined distance Mth, but it is not limited to that.Alternatively, for example, the transfer voltage Vtr may be obtainedwhen the temperature of the transfer part 30 is higher than thepredetermined temperature. With this configuration, printing isperformed continuously for a long period of time and when thetemperature of the transfer part 30 becomes higher than thepredetermined temperature, the computation part obtains the transfervoltage Vtr. With this, the image quality can be improved in the samemanner as the aforementioned embodiment. Herein, the temperature of thetransfer part may be measured at any surface of, inside, or in thevicinity of the transfer part 30. Other temperature surrounding thetransfer part may be available for the temperature.

Modified Embodiment 1-4

In the aforementioned embodiment, the cutting part 24 cuts the recordingmedium 9 to attain a state in which no recording medium 9 is present inthe transfer part 30, but not limited to that. For example, it becomesin a state in which no recording medium 9 is present in the transferpart 30 even when the rolled sheet is replenished when there is norecording medium 9 in the rolled sheet feeder 21. Therefore, theaforementioned technology may be applied in such a case.

Modified Embodiment 1-5

In the aforementioned embodiment, when the current measuring part 56 bof the high voltage power supply part 66 measures the current, thecurrent may be detected a plurality of times while changing the rollerangle of the secondary transfer roller 32. Hereinafter, an image formingapparatus according to this modified embodiment will be explained indetail.

FIG. 16 shows one example of a control mechanism of the image formingapparatus. The image forming apparatus is equipped with a high voltagepower supply part 66. The high voltage power supply part 66 is equippedwith an angle management part 56 c. The angle management part 56 c isconfigured to manage the roller angle of the secondary transfer roller32. Specifically, the angle management part 56 c is configured to managethe roller angle of the secondary transfer roller 32 so that the currentmeasuring part 56 b measures the current under a plurality of conditionsin which the roller angles of the secondary transfer roller 32 aredifferent.

Hereinafter, the operation when measuring the current Itr2 after thelabel 9 a of the recording medium 9 reaches the transfer part 30 in theflow for calculating the medium resistance value Rb will be exemplifiedfor explanation.

FIGS. 17A to 17D show roller angles of the secondary transfer rollers32. In this example, first, as shown in FIG. 17A, the current Itr2 ismeasured when the label 9 a of the recording medium 9 has reached thetransfer part 30. Next, as shown in FIG. 17B, the current Itr2 ismeasured when the secondary transfer roller 32 rotates by 90 degreesfrom when the label 9 a of the recording medium 9 has reached thetransfer part 30 (FIG. 17A). After that, the current Itr2 is measuredwhen the secondary transfer roller 32 rotates by 180 degrees from whenthe label 9 a of the recording medium 9 has reached the transfer part 30(FIG. 17A) and the current Itr2 is measured when the secondary transferroller 32 rotates by 270 degrees from when the label 9 a of therecording medium 9 has reached the transfer part 30 (FIG. 17A). In thisway, in this example, the current Itr2 is measured four times whilechanging the roller angle of the secondary transfer roller 32.

FIG. 18 shows a flowchart of the operation to calculate the mediumresistance value Rb.

First, the medium detection sensor 22 detects the recording medium 9(S61) and next, the label detection sensor 25 detects the label 9 a ofthe recording medium 9 (S62). Then, the current measuring part 56 b ofthe image forming apparatus 1 detects the current Itr1 before the label9 a of the recording medium 9 reaches the transfer part 30 (S63). Theseoperations are the same as those in Steps S41 to S43 of FIG. 13.

Next, the current measuring part 56 b detects the current Itr2 after thelabel 9 a of the recording medium 9 reaches the transfer part 30 (S641).That is, the current measuring part 56 b detects the current Itr2 in thestate shown in FIG. 17A. Then, the computation part 57 calculates theresistance values Rt1 and Rt2 of the transfer part 30 and calculates themedium resistance value Rb1 (S642). These operations are the same asthose in Steps S44 to S46 of FIG. 13.

Next, the current measuring part 56 b detects the current Itr2 after thesecondary transfer roller 32 rotates by 90 degrees (S651). That is, thecurrent measuring part 56 b detects the current Itr2 in the state shownin FIG. 17B. Then, the computation part 57 calculates the resistancevalues Rt1 and Rt2 of the transfer part 30 and calculates the mediumresistance value Rb2 (S652). These operations are the same as those inSteps S44 to S46 of FIG. 13.

Next, the current measuring part 56 b detects the current Itr2 after thesecondary transfer roller 32 rotates by 180 degrees (S661). That is, thecurrent measuring part 56 b detects the current Itr2 in the state shownin FIG. 17C. Then, the computation part 57 calculates the resistancevalues Rt1 and Rt2 of the transfer part 30 and calculates the mediumresistance value Rb3 (S662). These operations are the same as those inSteps S44 to S46 of FIG. 13.

Next, the current measuring part 56 b detects the current Itr2 after thesecondary transfer roller 32 rotates by 270 degrees (S671). That is, thecurrent measuring part 56 b detects the current Itr2 in the state shownin FIG. 17D. Then, the computation part 57 calculates the resistancevalues Rt1 and Rt2 of the transfer part 30 and calculates the mediumresistance value Rb4 (S672). These operations are the same as those inSteps S44 to S46 of FIG. 13.

Then, the computation part 57 calculates the medium resistance value Rbbased on the medium resistance values Rb1 to Rb4 obtained in Steps S642,S652, S662, and S672 (S68). Specifically, the computation part 57, forexample, calculates the medium resistance value Rb by calculating theaverage value of the four medium resistance values Rb1 to Rb4.

With the above, the flow of the calculation of the medium resistancevalue Rb is completed.

In this way, in the image forming apparatus, since the current isdetected a plurality of times while changing the roller angle of thesecondary transfer roller 32, for example, the resistance value of theurethane rubber layer 32 b of the secondary transfer roller 32 is notconstant and even when they are different according to the roller angle,the effects can be suppressed. As a result, in the image formingapparatus, since the accuracy of the calculation of the transfer voltageVtr can be improved, the image quality can be improved.

2. Embodiment 2

Next, an image forming apparatus 2 according to a second embodiment willbe described. This embodiment is configured to form an image on a labelin which the label width W is not constant. In addition, the samesymbols are allotted to the components that are essentially the same asthe components of the image forming apparatus 1 according to theaforementioned first embodiment and the explanations will be omitted.

As shown in FIG. 1, the image forming apparatus 2 has the same structureas the image forming apparatus 1. This image forming apparatus 2 isconfigured to form an image on a recording medium 8.

FIG. 19 shows one configuration example of the recording medium 8. Therecording medium 8 includes a label 8 a and a mount 8 b. The label 8 ahas a trapezoidal shape in this embodiment. The label 8 a has sides H1and H2 parallel to each other. The transfer part 30 performs thetransfer sequentially from the left side of the label 8 a (side H1). Thelabel width of the left end of the label 8 a (side H1) is W (0).

FIG. 20 shows one example of the control mechanism of the image formingapparatus 2. The image forming apparatus 2 is equipped with ameasurement position management part 67 and a control part 69. Themeasurement position management part 67 is configured to manage theposition for measuring the current within the label 8 a. Specifically,the measurement position management part 67, as explained later, isconfigured to manage the position for measuring the current in the label8 a so that the current measuring part 56 b measures the current atvarious positions in the label 8 a. The control part 69 is configured tomanage the overall operation of the image forming apparatus 2.

FIG. 21 shows a flowchart of the operation to determine the transfervoltage Vtr.

When the power of the image forming apparatus 2 is turned on, the imageforming apparatus 2 obtains the electrical characteristics of thetransfer part 30 (S71), receives print data (S72), calculates thetransfer voltage Vtr (S73), and starts the print operation (S74). Theseoperations are the same as those in Steps S1 to S4 according to thefirst embodiment (such as FIG. 9).

Next, the image forming apparatus 2 calculates the medium resistancevalue Rb and the label width W (S75). Specifically, the image formingapparatus 2, as will be explained later, calculates the mediumresistance value Rb using the first label 8 a among the plurality oflabels 8 a arranged side by side in the recording medium 8 andcalculates the label width W of the labels 8 a at predeterminedintervals. That is, since the label width W of the label 8 a is notconstant, the image forming apparatus 2 calculates the label width W atpredetermined intervals.

Next, the control part 69 of the image forming apparatus 2 confirmswhether or not the print distance M in the recording medium 8 afterstarting printing in Step S74 is longer than the predetermined distanceMth (for example, 1 m) (M>Mth) (S76). When the print distance M islonger than the predetermined distance Mth (“Y” in S76), the controlpart 69 confirms whether or not the label 8 a is present in the transferpart 30 and that the image forming apparatus 2 is in the non-formingstate in which no image is being formed (S77). Then, when it becomes ina state in which the label 8 a is present in the transfer part 30 andthe image forming apparatus 2 is not forming images (“Y” is S77), theimage forming apparatus 2 calculates the transfer voltage Vtr again(S78). At that time, the image forming apparatus 2 calculates thetransfer voltage Vtr using the second and later labels 8 a among theplurality of labels 8 a arranged side by side in the recording medium 8.

(Calculation of Medium Resistance Value Rb and Label Width W)

FIG. 22 shows a flowchart of the operation to calculate the mediumresistance value Rb and the label width W.

First, the medium detection sensor 22 detects the recording medium 8(S81) and next, the label detection sensor 25 detects the first label 8a of the recording medium 8 (S82). Then, the current measuring part 56 bof the image forming apparatus 2 detects the current Itr1 before thelabel 8 a of the recording medium 8 reaches the transfer part 30 (S83).Next, the current measuring part 56 b detects the current Itr2(0) afterthe label 8 a of the recording medium 8 has reached the transfer part 30(S84). Then, the computation part 57 calculates the resistance valuesRt1 and Rt2 of the transfer part 30 (S85) and calculates the mediumresistance value Rb3 (S86). These operations are the same as those inSteps 41 to 46 (FIG. 13) of the aforementioned first embodiment.Further, in Step S86, the computation part 57 performs a calculationusing the label width W (0) shown in FIG. 19. That is, since the currentItr2(0) is a current value when the vicinity of the side H1 of the label8 a is in the transfer part 30, the computation part 57 performs thecalculation using the label W(0) in the vicinity of the side H1.

Next, the measurement position management part 67 sets the variable m to1 (m=1) (S87). Then, the control part 69 controls the motor drive part54 and carries the recording medium 9 along the carrying path 20 by thepredetermined distance ΔM (for example, 10 mm) only (S88).

Next, the label detection part 50 confirms whether or not the label 8 ahas passed the transfer part 30 (S89). When the label 8 a has passed thetransfer part 30 (“Y” in S89), the flow is completed.

In Step S89, when the label 8 a has not passed the transfer part 30 (“N”in S89), the current measuring part 56 b detects the current Itr2 (m)(S90). That is, in Step S74, the image forming apparatus 2 has alreadystarted the print operation, and the voltage generation part 56 a issupplying the transfer voltage Vtr to the secondary transfer roller 32via the resistance element 39. Therefore, the current measuring part 56b detects the current Itr2 (m) flowing according to the transfer voltageVtr when the label 8 a of the recording medium 8 is present in thetransfer part 30. The current Itr2 (m) is a current value measured at aposition m×ΔM away from the side H1 of the label 8 a.

Next, the computation part 57 calculates the label width W (m) at theposition using the following formula based on the current Itr2 (m)detected in Step S90 (S91). The label width W (m) is obtained using theformula (11) and shown as follows.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 23} \right\rbrack & \; \\{{W(m)} = \frac{{a \times {Vs}\; 0 \times L} - {b \times L} - {{Itr}\; 2(m)}}{{a \times {Vs}\; 0} + b - J}} & (23)\end{matrix}$

For example, when the current Itr2 (m) detected in Step S90 isapproximately equal to the current Itr2 (0) detected in Step S84, thelabel width W (m) is approximately equal to the label width W (0).

Next, the measurement position management part 67 increments thevariable m (S92) and returns to Step S88. Then, Steps S88 to S92 arerepeated until the label 8 a passes the transfer part 30.

In this way, the image forming apparatus 2 calculates the mediumresistance value Rb and the label width W (m) for each predeterminedinterval using the first label 8 a of the recording medium 8.

(Calculation 2 of Transfer Voltage Vtr)

Then, the image forming apparatus 2 calculates the transfer voltage Vtrusing the second or later labels 8 a as will be explained below.

FIG. 23 shows a flowchart of the operations to calculate the transfervoltage Vtr.

First, the label detection sensor 25 detects the next label 8 a of therecording medium 8 (S101) and after the label 8 a has reached thetransfer part 30, the current measuring part 56 b detects the currentItr3 (S102).

Next, the computation part 57 calculates the resistance value Rt5 (S103)of the transfer part 30 in a state in which the label 9 a of therecording medium 9 is present in the transfer part 30 and the resistancevalue Rt6 of the transfer part 30 when no label 8 a of the recordingmedium 8 is present in the transfer part 30 to obtain the shaft voltageVs0 (S105). The operations of Steps S102 to S105 are the same as thosein Steps S51 to S54 (FIG. 15) of the aforementioned first embodiment.Further, in Steps S104 and S105, the computation part 57 performscalculations using the label width W (0) as shown in FIG. 19. That is,since the current Itr 3 is a current value when the vicinity of the sideH1 of the label 8 a is in the transfer part 30, the computation part 57performs the calculation using the label width W (0) in the vicinity ofthe side H1.

Next, the measurement position management part 67 sets the variable m to0 (m=0) (S106). Then, the computation part 57 calculates the transfercurrent Itr (m) (S107) and calculates the transfer voltage Vtr (m) to begenerated by the voltage generation part 56 a (S108). The operations ofSteps S107 and S108 are the same as those in Steps S55 and S56 (FIG. 15)of the aforementioned first embodiment. Further, in Steps S107 and S108,the calculations are performed using the label width W (m). Here, when mis 0, the label width is W(0).

Next, the control part 69 controls the motor drive part 54 to carry therecording medium 9 along the carrying path 20 for the predetermineddistance ΔM (for example, 10 mm) only (S109). Next, the label detectionpart 50 confirms whether or not the label 8 a has passed the transferpart 30 (S110). When the label 8 a has not passed the transfer part 30(“N” in S110), the measurement position management part 67 incrementsthe variable m (S111) and returns to Step S107. Then, Steps S107 to S111are repeated until the label 8 a passes the transfer part 30.

In Step S110, when the label 8 a has passed the transfer part 30 (“Y” inS110), the control part 69 confirms whether or not printing is completed(S112). When printing continuously on the label 8 a (“N” in S112), itreturns to Step S101 and the process is performed on the next label 8 a.Further, when the printing is completed (“Y” in S112), the flow iscompleted.

In this way, the image forming apparatus 2 is configured to obtain thelabel width W (m) at predetermined intervals in the label 8 a andcalculate the transfer voltage Vtr (m) based on the label width W (m) ateach position. With this, even when forming an image on a label 8 a inwhich the label width W (m) is not constant, the current density of thecurrent flowing through the recording medium 9 can be brought closer tothe medium current density Jp and the potential difference between thevoltage of the surface and the voltage of the back surface of therecording medium 9 can be brought closer to the medium voltage Vp. As aresult, excellent transfer characteristics can be obtained and the imagequality can be improved in the image forming apparatus 2.

Further, since the image forming apparatus 2 is configured to calculatethe label width W (m) based on the current Itr2 (m), there is no need toequip a sensor exclusively for detecting the label width W (m), andtherefore the configuration can be simple.

As described above, since this embodiment is configured to obtain thelabel width of the label at predetermined intervals to calculate thetransfer voltage based on the label width at each position, the imagequality can be improved even when forming an image on a label in whichthe label width is not constant.

Modified Embodiment 2-1

In the aforementioned embodiment, the labels 8 a has a trapezoidalshape, but not limited to that, and the labels can be of any shape, suchas, e.g., a circular shape, an elliptical shape, and a star shape.

Modified Embodiment 2-2

In the aforementioned embodiment, the label width W (m) is calculatedusing the first label 8 a among the labels 8 a arranged side by side andthe transfer voltage Vtr (m) is calculated using the second label 8 a orlater, but not limited to that. For example, the label width W (m) aswell as the transfer voltage Vtr (m) may be calculated using each of thelabels 8 a.

Modified Embodiment 2-3

In the aforementioned embodiment, the predetermined distance ΔM is setto 10 mm, for example, but not limited to that, and may be shorter than10 mm or longer than 10 mm, for example. The predetermined distance ΔM,for example, is preferably set according to the size and the shape ofthe label 8 a.

Modified Embodiment 2-4

In the aforementioned embodiment, the label width W (m) is obtained pera predetermined interval, but not limited to that, and the intervals forobtaining the label width W (m) may change.

Modified Embodiment 2-5

Each of the modified embodiments may be applied to the image formingapparatus 2 according to the aforementioned first embodiment.

The present invention was explained above with reference to someembodiments and modified embodiments, but the present invention is notlimited to these embodiments, etc., and various modifications arepossible.

For example, in the aforementioned embodiments, etc., printing isperformed on a rolled sheet, but not limited to that, and printing maybe performed on any medium as long as it is a long medium. Specifically,for example, a so-called continuous sheet provided with perforations perpredetermined length may be used.

Further, for example, in the aforementioned embodiments, the presentinvention is applied to a color printer, but not limited to that, andfor example, it may alternatively be applied to a monochromatic printer.

Furthermore, for example, in the aforementioned embodiments, the presentinvention is applied to a printer, but not limited to that.Alternatively, for example, the present invention may be applied to amultifunction peripheral apparatus (Multi Function Peripheral) havingfunctions of a printer, a FAX, a scanner, etc.

What is claimed is:
 1. An image forming apparatus, comprising: an imageforming part having a transfer part including a transfer member, and arotation member arranged so as to face the transfer member, and thetransfer part being configured to transfer a developer with a transfervoltage to a recording medium of which a front surface is composed witha transfer region and a non-transfer region arranged between thetransfer member and the rotation member, in the transfer target regionwhere the developer is to be disposed and in the non-transfer regionwhere the developer is not to be disposed; a measuring part thatmeasures a first electric resistance value and a second electricresistance value, the first electric resistance value being defined as aresistance value between the transfer member and the rotation member ina state in which the transfer region of the recording medium is notpresent between the transfer member and the rotation member, and thesecond electric resistance value being defined as another resistancevalue between the transfer member and the rotation member in anotherstate in which the transfer region of the recording medium is presentbetween the transfer member and the rotation member; and a control partthat determines the transfer voltage value in the transfer part based onthe first electric resistance value and the second electric resistancevalue, the measuring part measures the first electric resistance valuebefore the transfer part starts transferring the developer to therecording medium, the measuring part further measures a third electricresistance value that is defined as another resistance value between thetransfer member and the rotation member in another state in which thetransfer region of the recording medium is present between the transfermember and the rotation member, the second electric resistance value ismeasured after the third electric resistance value is measured, and thecontrol part takes the third electric resistance value into account todetermine the transfer voltage value.
 2. The image forming apparatusaccording to claim 1, wherein the transfer part sequentially transfersthe developer in a carrying direction along which the recording mediumis carried, and the measuring part measures the second electricresistance value after the transfer part transfers the developer to therecording medium for a predetermined length in the carrying direction.3. The image forming apparatus according to claim 1, further comprising:a temperature sensor that is disposed in the image forming apparatus anddetects a temperature of the transfer part, wherein the measuring partmeasures the second electric resistance value when the temperature ofthe transfer part is equal to or higher than a predeterminedtemperature.
 4. The image forming apparatus according to claim 1,wherein the measuring part measures the second electric resistance valuewhen the transfer region of the recording medium is present between thetransfer member and the rotation member and when the transfer part isnot transferring the developer to the transfer region of the recordingmedium.
 5. The image forming apparatus according to claim 1, wherein thecontrol part calculates a medium resistance value with respect to thetransfer region of the recording medium based on the first electricresistance value and the third electric resistance value, and thecontrol part determines the transfer voltage value based on the mediumresistance value and the second electric resistance value.
 6. The imageforming apparatus according to claim 1, further comprising: anenvironment detection part that detects one or both of an environmentaltemperature and an environmental humidity, wherein the control parttakes a detection result of the environment detection part into accountto determines the transfer voltage value.
 7. The image forming apparatusaccording to claim 6, wherein, based on the detection result of theenvironment detection part, the control part obtains a first targetvalue of a current density of a current to be flown through therecording medium, and a second target value of a potential differencebetween a voltage on the front surface of the recording medium and avoltage of a back surface of the recording medium, and the control partdetermines the transfer voltage value based on the first target valueand the second target value.
 8. The image forming apparatus according toclaim 1, wherein the measuring part measures the second electricresistance value multiple times across the transfer region of therecording medium.
 9. The image forming apparatus according to claim 8,wherein after measuring the second electric resistance value, themeasuring part measures the second electric resistance value again whenthe transfer region of the recording medium has passed through betweenthe transfer member and the rotation member for a predetermined length.10. The image forming apparatus according to claim 9, wherein themeasuring part measures the second electric resistance value every timewhen the transfer region of the recording medium passes through betweenthe transfer member and the rotation member for a predetermined length.11. The image forming apparatus according to claim 8, wherein thecontrol part obtains a width of the transfer region based on the secondelectric resistance value and determines the transfer voltage valuebased on the width of the transfer region.
 12. The image formingapparatus according to claim 1, wherein the transfer body rotates in apredetermined rotational direction, and the second electric resistancevalue includes an electric resistance value between the transfer memberand the rotation member, being determined when a rotation angle of thetransfer member is a first angle, and another electric resistance valuebetween the transfer member and the rotation member, being determinedwhen a rotation angle of the transfer member is a second angle that isdifferent from the first angle.
 13. The image forming apparatusaccording to claim 1, wherein the recording medium is a rolled sheetthat is configured with a continuous mount and labels attached on themount with an interval to each other, and the transfer regioncorresponds to each of the labels.
 14. The image forming apparatusaccording to claim 1, wherein the image forming part further includes atransfer belt on which the developer is carried, and the transfer membertransfers the developer on the transfer belt to the recording medium.15. The image forming apparatus according to claim 1, wherein therotation member includes a photosensitive member on which a latent imageis formed, and the transfer member transfers a developer image, which isformed with the developer, on the photosensitive member to the recordingmedium.
 16. An image forming method performed with a transfer part,comprising: measuring, in the transfer part having a transfer member anda rotation member arranged so as to face the transfer member andconfigured to transfer a developer with a transfer voltage to arecording medium of which a surface is composed with a transfer regionand a non-transfer region arranged between the transfer member and therotation member, a first electric resistance value between the transfermember and the rotation member in a state in which the transfer regionof the recording medium is not present between the transfer member andthe rotation member; measuring a second electric resistance valuebetween the transfer member and the rotation member in a state in whichthe transfer region of the recording medium is present between thetransfer member and the rotation member; measuring a third electricresistance value that is defined as another resistance value between thetransfer member and the rotation member in another state in which thetransfer region of the recording medium is present between the transfermember and the rotation member, the third electric resistance valuebeing measured before the second electric resistance value is measured,and determining the transfer voltage value in the transfer part based onthe first electric resistance value, the second electric resistancevalue and the third electric resistance value.